The art of surveying involves the determination of unknown positions, surfaces or volumes of objects or setting out of known coordinates using angle and distance measurements taken from one or more positions. In order to perform these measurements, a surveying device frequently is used comprising a distance measuring instrument with an integrated distance and angular measurement of the type referred to as a total station, i.e. comprising a combination of electronics and optics. A total station is furthermore provided with a computer or processing or control unit with writable information for measurements to be performed and for storing data obtained during the measurements. Preferably, the total station calculates the position of a target in a fixed ground-based coordinate system. A more detailed description of such a total station can for example be found in WO 2004/057269 by the same applicant.
In surveying, the use of a camera in a geodetic instrument such as a total station may provide for improved user convenience and new and improved functions. Particularly, a view such as an image or a video feed provided by the camera and shown on a display of the instrument may be used for assisting target selection and for providing the user with an overview of potential points of interest. For a general description of a geodetic instrument comprising a camera for capturing an image or a video feed of the view towards which the geodetic instrument is aimed, reference is made to WO 2005/059473.
Furthermore, the tracker system may also utilize a camera to track targets having reflective prisms, a so called camera-based tracker. Such a camera-based tracker includes a coaxial optical radiation source and receiving optics. In order to track or detect the target, two consecutive images are captured as close in time as possible. The first image is captured with the transmitter activated or switched on such that an area including the target is illuminated and the second is captured with the transmitter switched off. Accordingly, the first image captures objects within the area and the reflection from the reflective prism of the target (i.e. the reflected light from the prism) and the second image captures the objects within the area. Subsequently an image differentiating process is performed in which a differential image based on the two images is created and all non-reflective objects are cancelled out from the differential image. Thereby, an image including only the reflective target is obtained which can be used by the tracker when tracking the target.
However, if other reflective objects are present within the illuminated area such as traffic signs, reflective vests worn by persons working within the area, car lights, etc., the first image will in addition to the reflected light from the reflective prism also contain reflected light from the other reflective objects present within the illuminated area. Thus, the differential image will contain the reflective target as well as the other reflective objects. This will cause problem for the tracker since there is more than one object to track in the image, which in turn may lead to that the tracker loses track of the target (i.e. loses lock on the target) or that the aiming will become incorrect.
Hence, there is a need within the art for an improved tracker that is capable of tracking reflective prisms also in the presence of other reflective objects such as traffic signs, reflective vests worn by persons working within the area, car lights, etc.
There is also a need within the art for an improved tracker that is capable of tracking a movable object such as a reflective prism arranged on a vehicle such as an excavator in the presence of other reflective objects and which tracker is capable of distinguishing that specific object from other movable objects.